1. Field of the Invention
The present invention relates to a distributed feedback laser device mainly employed for optical fiber communication.
2. Description of the Background Art
A distributed feedback laser device employed for optical fiber communication has a diffraction grating in a cavity and oscillates at an oscillation wavelength corresponding to the cycle of this diffraction grating, thereby stably operating in a single vertical mode also in high-speed modulation. Therefore, the distributed feedback laser device is generally employed for optical fiber communication over a long distance or at a high bit rate. One of parameters remarkably influencing the characteristics of the distributed feedback laser device is a beam coupling coefficient. This parameter remarkably influences not only static characteristics such as the threshold current and slope efficiency of the laser device but also noise and dynamic characteristics.
For example, FIGS. 37, 38A and 38B show a conventional distributed feedback laser device 120 disclosed in Japanese Patent Laying-Open No. 2000-114652. In this distributed feedback laser device 120, (1) an n-conductivity type InP cladding layer 116, (2) InGaAsP light trap layers 117 and 118, (3) an active layer 119, (4) InGaAsP light trap layers 120 and 121, (5) a diffraction grating 106, (6) a p-conductivity type InP cladding layer 122 and (7) a p-conductivity type InGaAs contact layer 123 are successively stacked on an n-conductivity type InP substrate 115 in ascending order.
Light intensity of a laser beam oscillated from the distributed feedback laser device 120 along the thickness direction shown in FIG. 38A spreads vertically about the active layer 119, as shown in FIG. 38B. As shown in FIGS. 38A and 38B, the light intensity strongly depends on the height h of the diffraction grating 106, i.e., the amplitude of waves, and the distance H between the diffraction grating 106 and the active layer 119.
However, the height h of the diffraction grating 106 remarkably varies with dispersion in etching depth for forming the diffraction grating 106 etc. The diffraction grating 106 is etched by etching a small region of about 0.2 μm in width. Therefore, the etching rate is remarkably dispersed in the wafer plane with remarkable variation with fabrication, i.e., run-to-run variation. Consequently, the beam coupling coefficient varies with the magnitude of dispersion of the height h of the diffraction grating 106. In practice, further, influence is also exerted by thickness dispersion in crystal growth. If the LnGaAsP light trap layers 120 and 121, 118 and 120 or 117 and 121 are increased in thickness, for example, the distance H between the diffraction grating 106 and the active layer 119 is increased. Therefore, light intensity overlapping with the diffraction grating 106 is reduced to reduce the beam coupling coefficient. Thus, the beam coupling coefficient is remarkably dispersed in the distributed feedback laser device 120 shown in FIG. 37 due to small thickness variation in fabrication or refractive index variation resulting from composition variation, and it is difficult to fabricate the distributed feedback laser device 120 with an excellent yield.
FIG. 39 shows another conventional distributed feedback laser device 120 not influenced by the aforementioned dispersion of the height h of the diffraction grating disclosed in Journal of Lightwave Technology, Vol. 7 (1989), pp. 2072-2077, for example. In this distributed feedback laser device 120, (1) an InP cladding layer 122a, (2) an active layer, (3) an InP cladding layer 122b, (4) a diffraction grating 106 and (5) an InP cladding layer 122c are successively stacked on an InP substrate 115 in ascending order. The beam coupling coefficient of the distributed feedback laser device 120 shown n FIG. 39 is decided by two thicknesses dInP (the thickness of the InP cladding layer 122b) and dgrating, the refractive index of the diffraction grating 106 and the sectional shape of the diffraction grating 106. When the aforementioned diffraction grating 106 has grating bars arranged at a prescribed pitch, therefore, the beam coupling coefficient does not depend on the aforementioned etching depth. Therefore, the distributed feedback laser device 120 has no factor corresponding to the height h of the diffraction grating 106, the main factor for dispersion of the beam coupling coefficient in the distributed feedback laser device 120 shown in FIG. 37. Thus, dispersion of the beam coupling coefficient in the distributed feedback laser device 120 shown in FIG. 39 is smaller than that in the distributed feedback laser device 120 shown in FIG. 37.
However, the thicknesses and the refractive index decided in film forming steps are strongly influenced by dispersion of growth rates in film forming apparatuses, compositions at film forming opportunities and in-plane distribution of growth rates and compositions specific to the film forming apparatuses. The beam coupling coefficient is dispersed at film growth opportunities and in the wafer plane due to influence exerted by the aforementioned dispersion.
In the following description, the wording “film forming” includes both of (g1) a case of growing an epitaxial film matching with an underlayer film in crystal orientation and (g2) a case of forming a crystal film or an amorphous film with no regard to matching in crystal orientation. The latter case (g2) corresponds to deposition of a polycrystalline film or the like.
FIG. 40 shows results of calculation of deviation of the beam coupling coefficient from a design value resulting from variation of the thickness dInP. FIG. 41 shows results of calculation of deviation of the beam coupling coefficient from the design value resulting from variation of the thickness dgrating. FIG. 42 shows results of calculation of deviation of the beam coupling coefficient from the design value resulting from deviation of the refractive index of the diffraction grating. It is understood from these results of calculation that the beam coupling coefficient varies by ±14 to 25% due to thickness variation of ±25% or refractive index variation of 1%. The characteristics of the conventional distributed feedback laser device are remarkably dispersed due to such dispersion of the beam coupling coefficient, to hinder improvement of the fabrication yield.